1. Field of the Invention
This invention pertains to the field of superconducting tunnel junctions and more specifically relates to the field of superconducting tunnel junctions having deposited barriers.
2. Description of the Prior Art
Many metals will exhibit the superconducting state under certain critical parameters, which are all temperature dependent, having their largest numerical values at 0 K. and vanishing at the transition temperature T.sub.c. The transition temperature is a constant of the material and varies from a range of 0 K. to approximately 24 K. Niobium is a material of particular interest because of its refractory properties and its relatively high transition temperature T.sub.c of 9.4 K. This means that when cooled with liquid hydrogen to a temperature of approximately 4 K. it will be well below the transition temperature. When the conditions for superconductivity are obtained, the material provides perfect conductivity. However, as the temperature approaches T.sub.c, the superconductive properties vanish. In the superconducting state, the electrons form bond pairs. When the critical current of the superconductors is reached, the kinetic energy of the pairs becomes sufficient to result in pair breaking, whereby single electrons are released which scatter, leading to electrical resistance. This will also occur as the transition temperature T.sub.c is reached.
A metal tunnel junction consists of a sandwich of two metal electrodes separated by a thin insulating barrier. While the insulating barrier prohibits bulk state conductivity, it does permit electron transport from one electrode to the other by the tunneling phenomenon. At room temperatures, the conductance of the junction is substantially linear. However, as the junction is cooled below T.sub.c of the electrodes, the current-voltage (I-V) characteristic of the junction becomes non-linear as a result of having developed an energy gap in which there are no available electron states; as a result no current will flow across the junction until the voltage impressed across the junction is equal to the sum of the energy gaps, at which time the electrons can tunnel from one electrode to states above the gap in the other electrode. Such a device, at a temperature below T.sub.c, can then be current biased in the zero voltage state and caused to switch under the influence of an input current or magnetic field between the zero voltage state and the resistive state.
Superconducting tunnels junctions are known in the art and are used in both Josephson devices and S-I-S (superconductor-insulator-superconductor) microwave and millimeter wave detectors and mixers. Josephson junctions comprise, for example, two superposed layers of superconductive material with an insulator or semiconductor layer constituting a barrier therebetween, whereby current flows from one superconductive layer to the other through the insulating barrier via the Josephson electron pair tunneling effect. With the superconductive layers connected into a superconductive loop and control lines disposed adjacent the junction, the D.C. Josephson zero voltage current flowing through the device may be controlled so as to provide the necessary current steering control function in the Josephson circuitry. Detectors and mixers have also been devised, based on the quasi-particle (single electron) tunneling currents in superconducting tunnel junctions.
Very important to the use of Josephson junction devices, particularly in logic circuits, are its electrical characteristics, namely, the value of V.sub.m, which is a product of the critical current I.sub.c flowing through a junction and the subgap resistance R.sub.s. The critical current I.sub.c can be defined as the maximum superconducting current that can flow through the Josephson junction, which if exceeded will result in the junction losing its superconducting property, permitting a voltage drop across the junction. The subgap resistance R.sub.s is the resistance of the junction measured at a voltage less than the sum of the energy gaps of the superconducting electrodes.
The gap energy is the engergy required to disassociate an electron pair and can be supplied by thermal excitation, electric fields, or magnetic fields. When the gap energy is exceeded, the superconducting electrodes revert to the normal metallic state, in which current is manifested by the transfer of single electrons, rather than electron pairs.
It is generally recognized that the higher the value of V.sub.m, the more ideal the behavior of the Josephson junction in many digital circuits. That is, the device tends to approach the theoretical current-voltage (I-V) characteristic of an ideal superconducting tunnel junction, in which the leakage current below the energy gaps would vanish at a temperature of absolute zero (T=0 K.).
Tunneling from an electrode to a localized state within the barrier material provides an internal conductive shunt across the junction. To the extent that localized states contribute to conduction, the junction is considered to be non-ideal. When conduction is due to electrode-to-electrode tunneling alone, important properties of the junction, such as the superconducting energy gap, may be determined.
Another figure of merit is the product of the critical current I.sub.c and the normal resistance R.sub.n. R.sub.n is the resistance of the device at voltages greater than the sum of the energy gaps of the superconducting electrodes.
A further figure of merit for Josephson devices is the steepness of the rise in quasi-particle current at the sum of the superconducting energy gaps. The steeper the rise, the more useful the device may be in a digital circuit, as noted heretofore. In a superconductor the phenomenon of pairing of electrons is responsible for the lossless supercurrent in the metal electrodes and a Josephson or electron-pair current in S-I-S tunnel junctions. The energy or applied voltage required to break the pair is referred to as the superconducting energy gap. Such pairs when broken create single particle excitations known as quasi-particles. Sufficiently great thermal energy, magnetic fields, or voltage drops across the tunneling junction can break these pairs and create the quasi-particles. As an example, in the prior art certain power supply circuits have been designed for use with latching Josephson logic in which the supply voltage is set by the voltage limiting action of the jump in quasi-particle current. The steeper the step in quasi-particle current at V.sub.g, the voltage of the sum of the superconducting energy gaps of the electrodes (see FIG. 5), the more accurate such a voltage regulator will be. [J. Matisoo, IBM J. Res. Develop. Vol 24, No. 2, P. 123 (1980)] Moreover, the prior art studies of the Josephson and quasi-particle devices in applications as very-low-noise detectors and mixers for millimeter wavelengths favor the quasi-particle devices, and based on quantum mode detection theory indicates that a lower frequency operation and conversion gain may be obtained. It is suggested that the availability of conversion gain from a photon-noise-limited mixer would make it appear possible that the performance of millimeter-wave S-I-S heterodyne receivers will approach the quantum noise limit. Thus the prior art shows the advantage for devices with significant improvement in quasi-particle current characteristics [P. L. Richards and T. M. Shen, IEEE Trans. Electron Devices, Vol. ED.-27, P. 1909 (1980)]
U.S. Pat. No. 4,176,365, assigned to the Assignee of the present invention, has contemplated using relatively uniform deposited hydrogenated amorphous silicon barriers in cooperation with niobium superconductive electrodes. However, it has been found that interdiffusion or alloying of the electrode material and the hydrogenated semiconductor material could lower the superconducting energy gap adjacent to the barrier, resulting in a reduction of the figure of merit V.sub.m, and decreased subgap resistance R.sub.s. It is observed that the hydrogen impregnation results in "poisoning" the niobium superconducting electrodes; that is, results in a reduction of the superconducting transition temperature, such that the predicted advantages from the reduction of localized states with hydrogenation are not realized.
The present invention makes it possible to obtain the advantages of hydrogenation, which affords unusually high current density, without attendant loss in the "figures of merit", by providing a composite barrier structure where the region adjacent to the electrodes is free from hydrogen, while the central region of the barrier is hydrogenated in a predetermined amount, such that higher values of V.sub.m and R.sub.n and a more abrupt rise in quasi-particle current at the sum of the energy gaps are observed.